An engine for driving pumps is provided with a main pump and a fan pump. The main pump is for excavation or other work and serves to drive a working unit system and a turning system of a hydraulic shovel. The fan pump serves to perform variable control of the pump discharge rate of a fan pump by system of an electro-hydraulic transducing valve so as to control the revolution speed of the fan motor that is adapted to drive a cooling fan for cooling an intake air cooler, an oil cooler, and a radiator. Through the control of the revolution speed of the fan motor, the fan pump serves to control the fan revolution speed of the cooling fan. The electro-hydraulic transducing valve is controlled by a controller. Temperature sensors respectively detect temperatures of the intake air, the hydraulic oil, and the coolant, all of which are cooled by the cooling fan. The controller serves to control the fan revolution speed of the cooling fan by controlling the flow rate of the hydraulic oil fed from the fan pump so that the detected temperatures are brought to the same level as the respective target temperatures that have been set beforehand (refer to Japanese Patent No. 3295650 as an example).
Japanese Patent No. 3295650 mentioned above describes an automatic engine speed control system (hereinafter referred to as “AEC”) for automatically reducing the engine speed to a given, low revolution speed and a one-touch low idling switch to be operated by an operator with a single operation so as to put the engine in the one-touch low idling state, in which the engine speed is controlled at a given, low speed. When the levers are at the neutral position, the hydraulic actuators are prevented from being operating. Should the engine be in either one of the aforementioned two states when the levers are at the neutral position, i.e. the AEC state when AEC is on or the one-touch low idling state, the engine speed is lower than in cases where the working unit is in operation. Regardless of the reduction in the engine speed, however, there is virtually no decrease in the fan speed, because the controller controls the fan revolution speed of the cooling fan by controlling the pump discharge rate of the fan pump so as to bring the detected temperature of each cooling target fluid, such as the hydraulic oil, to the same level as each respective target temperature determined beforehand. In other words, there is virtually no decrease in the amount of cooling air.
To summarize, the controller is adapted to control the fan revolution speed of the cooling fan by controlling the pump discharge rate of the fan pump so as to bring the detected temperature of each cooling target fluid, such as the hydraulic oil, to the same level as each respective target temperature determined beforehand. Therefore, when the detected temperature of a cooling target fluid, such as the hydraulic oil, is high, the controller controls the fan revolution speed of the cooling fan at a high speed.
Should the levers be returned to the neutral position during heavy load operation, the controller reduces the engine speed by system of either the AEC control or the one-touch low idling control while controlling a capacity changing system, such as a swash plate, of the variable delivery pump that serves to feed the hydraulic oil to the hydraulic actuators, thereby reducing the pump discharge rate of the variable delivery pump. As a result, the flow rate of the hydraulic oil fed to the hydraulic actuators is reduced sharply, resulting in sharp decrease of the flow rate of the oil returned to the tank from the hydraulic actuators through the oil cooler.
This may cause thermal strain resulting from the hydraulic oil in the oil cooler being rapidly cooled by the cooling fan rotating at a high speed to cope with the high temperature of the hydraulic oil. Such thermal strain presents the possibility of breakage of or other damage to the oil cooler.
In order to solve the above problems, an object of the invention is to provide a fan revolution control method that is capable of improving the durability of a cooling system provided with a cooling fan by reducing thermal strain that occurs in such cooling system.